1. Field of the Invention
The present invention generally relates to an organic electro-luminescent (EL) device (also known as OELD) and a method for fabricating the same, and more particularly to a white light emitting organic EL device able to directly emit a continuous full-color light providing three different frequency bands and a method for fabricating such an organic electro-luminescent device.
2. Description of the Prior Art
The organic electro-luminescent device has attracted tremendous attention due to its advantages over other display panels. These advantages include self-luminescence, large visual angle, short response time, compact size, light weight, reduced dimension in thickness, high brightness, low power consumption, simple fabrication, and the ability for light emitting in a full-color range. Therefore, such an organic electro-luminescent device is increasingly required to replace the currently used white light sources such as fluorescent lamps and light bulbs to save energy, and the technologies thereof have widely been studied in the industry all over the world.
Please refer to FIG. 1, which is a cross-sectional view showing the structure of a conventional organic EL device in accordance with the prior art. The organic EL device is characterized in that a bottom electrode 13 is formed on a substrate 11, and on the bottom electrode 13 there are formed an organic layer 15 comprising a hole injection layer, a hole transport layer, an organic emitting layer or an electron transport layer, an opposed electrode 17 and an sealing cap layer 19 by evaporation in turn. Due to the limitation of the material used in the organic layer 15, such a device as described in FIG. 1 is only able to emit monochromatic lights, preferably blue, red, or green according to various chosen fluorescent materials, and fail to achieve the object of emitting a white light or continuous full-color light. Therefore, several modifications have to be made on the device structure to obtain a white light source.
For example, Forrest et al. (The Trustees of Princeton University, Princeton, N.J.) discloses “Multicolor organic light emitting devices” as U.S. Pat. No. 5,757,026 and Chao, et al. (Industrial Technology Research Institute, Hsinchu, TW) discloses “Method for fabricating an organic electro-luminescent device” as U.S. Pat. No. 6,037,190. Please refer to FIG. 2, which is a cross-sectional view showing the structure of a conventional white light emitting organic EL device similar to the organic EL device disclosed in U.S. Pat. No. 6,037,190. In FIG. 2, the white light emitting organic EL device is characterized in that three organic layers 25G, 25R and 25B for emitting a green light, a red light and a blue light, respectively, are interposed between the bottom electrode 13 and a corresponding opposed electrode 17. Therefore, a white light source is obtained by mixing the lights from the three organic layers.
Nevertheless, there exist some problems in the aforementioned white light emitting organic EL device, in that:                (1) the organic material for emitting a red light has a shorter lifetime than other organic materials, which makes the white light emitting organic EL device mal-functional because the organic material for emitting a red light has failed;        (2) the manufacturing processing for forming different organic layers on a chip is difficult to carry out; and        (3) hardware implementation is difficult because the voltages for activating different organic layers are different.        
In order to avoid the problems resulting from the organic EL device in FIG. 2, there have been proposed a number of disclosures such as U.S. Pat. No. 6,008,578 “Full-color organic electro-luminescent device with spaced apart fluorescent areas” filed by Chen (Hsinchu, TW), U.S. Pat. No. 5,294,870 “Organic electro-luminescent multicolor image display device” filed by Tang, et al. (Eastman Kodak Company, Rochester, N.Y.) and U.S. Pat. No. 5,717,289 “Thin film electro-luminescent element easily regulating emitted light to white” filed by Tanaka (NEC Corporation, Tokyo, JP). Please refer to FIG. 3, which is a cross-sectional view showing the structure of a conventional white light emitting organic EL device similar to the thin film electro-luminescent element disclosed in U.S. Pat. No. 5,717,289. In FIG. 3, the white light emitting organic EL device is characterized in that a color conversion layer 30 is interposed between the bottom electrode 13 and the substrate 11. The color conversion layer 30 is formed of a fluorescent material. When a current is injected from the bottom electrode 13 into the opposed electrode 17, the organic layer 15 emits a blue light (B). As the blue light (B) passes through the color conversion layer 30, a certain percentage of the blue light (B) turns out to be red (R) and green (G). Therefore, a white light source is obtained.
However, as shown in FIG. 3, since the color conversion layer 30 is interposed between the bottom electrode 13 and the substrate 11, the flatness of the subsequent organic layer 15 is adversely affected and so is the lifetime of the organic layer 15.
Moreover, one modification of the structure shown in FIG. 3 is to form the color conversion layer 30 on the organic layer 15. In other words, the organic layer 15 is formed prior to the formation of the color conversion layer 30. As a result, the organic layer 15 may be damaged due to the high temperature as the color conversion layer 30 is formed by evaporation, leading poor full-color display quality.
Therefore, in view of the aforementioned problems, there is need in providing a white light emitting organic EL device and a method for fabricating such an organic EL device, resulting to simplify the fabrication processing and achieve longer lifetime as well as better full-color display quality of the device.